Method of coating paper and composition therefor



Patented Aug. 5, 1947 METHOD OF COATING PAPER. AND COIVIPOSITIONTHEREFOR Gilbert K. Dickerman and Robert W. Riley, Wisconsin Rapids,Wis., assignors to Consolidated Water Power & Paper Company, WisconsinRapids, Wis., a corporation of Wisconsin No Drawing. Application January24, 1944, Serial No. 519,542

13 Claims. 1

This is a continuation in part of our co-pending application for patent,Serial N 0. 439,766, filed April 20, 1942, which is a continuation inpart of our application, Serial No. 280,732, filed June 23, 1939. Thepresent invention relates particularly to an improved paper coatingcomposition of the mineral type for forming a mineral coated surfaceupon paper which is receptive to fine printing, and a method of usingsuch coating material in a machine or roll coating operation.

It is well known that the printing quality of a sheet of paper isdependent upon the uniform flatness and character of the printingsurface. In some machine coating operations of the roll coating type,uniformity is different of attainment. However, because of the speed ofsuch processes and the economies effected thereby, imperfections in thecoated surface have been heretofore tolerated.

Typical machine coating operations of the roll coating type, to whichthis invention is adapted, are exemplified by the process set forth inthe patents to Peter J. Massey, Nos. 1,921,368 and 1,921,369, and theapparatus of the Massey et al., Patent No. 2,105,981.

The Massey coating process is potentially operable at relatively highspeeds, up to 1,000 linear feet per minute web speed or higher, asopposed to conventional coating methods heretofore practiced where themaximum web speed seldom exceeds 200 to 400 feet per minute. As aconsequence the Massey process is capable of being performed inconjunction with, and as a step in,

the process of making paper, or more strictly speaking, making coatedpaper, as opposed to coating paper which has been previously separatelymade.

It is an object of the present invention to provide a coatingcomposition particularly adapted to take the fullest advantage of thehighest speed potentialities of the Massey or machine coatingoperations, and to enable the coating operation to effectively takeplace without sacrifice in character or quality of the resulting coatingformed on the paper, at high paper making speeds of from about 800 toabout 1,000 linear feet per minute.

For use in machine coating operations of the class herein contemplated,the coating material must be flowable in the sense that it can be ormetering rolls and transferable in controlled quantities from rollsurface to roll surface and from theapplicator or coating roll surfaceto the paper web surface in controlled amount and in such form as torequire no further working or smoothing after application to the websurface.

In addition to having the foregoing characteristics, the coatingcomposition of the present inventionenables the formation therefrom ofpreformed films, on the rolls of the coating apparatus, of relativelyhigh density which facilitates transfer to the paper web at such highspeeds. The composition is also adapted to be formed in relatively thinfilms of high solids content and to be coated on paper in a relativelyheavy weight at such high speeds, without the formation of ridges, whichridges would otherwise be formed with prior thicker films of converselygreater water content. Our low water content coating compositionadditionally contributes to the ability to effectively operate at highspeeds by reason of the fact that the excess of water that must berapidly absorbed by the sheet to prevent sticking on the dryers, is at aminimum, and of course the low moisture content minimizes the amount ofwater necessary to be evaporated from the sheet, with attendant addedadvantage in the operation cost. In all of those prior coating processeswherein the coating material, after having been initially applied to thesurface of the web, is brushed,

- scraped or subjected to an air stream traveling at relatively highvelocity to smooth or mold the adaptable for the reception of fineprinting, the coating material employed must be substantially liquid,the term being employed in its strict sense as connoting a substancewhich is substantially a true Newtonian liquid. In a true Newtonianliquid the rate of shear is directly proportional linearly to theshearing stress and the coating liquid has a zero yield point. It isthis characteristic of the coating materials heretofore employed whichrenders them suitable for after working" coating operations, that is,the type of coating operations hereinbefore described. If the coatingmaterials had an appreciable yield point or were of relatively lowmobility, they could not be smoothed on the web by brushing, scraping,etc.

The coating material of the present invention is a plastic orpseudo-plastic, having an appreciable yield point or apparent yieldpoint and being in general of lesser mobility than the liquid coatingmaterials heretofore employed. In our process of employing our coatingmaterial theexistence of an appreciable yield point or apparent yieldpoint is used to advantage in maintaining the coating materialrelatively immobile or static, so to speak, after it has been initiallyapplied to the web, since it is contemplated that inbefore described,must possess those transfer properties which exist in a coating materialwhich has a yield point or apparent yield point between predeterminedlimits and has a mobility within predetermined limits. Such a coatingmaterial can be transferred from roll surface to roll surface, or fromthe applicator roll surface to the web to be coated without streakingand without the aqueous content of the coating material, normallyrelatively low, from leaving the coatin too rapidly or remaining toolong in the coating material.

We have found that our coating material functions satisfactorily if, inaddition to its critical limitations of total solids and adhesivecontent, it has a yield point or apparent yield point between about 500and 5,200 dynes per square centimeter and has a range of mobilitybetween about 0.0575 to 0.750 rhe, (mobility being the rate of change ofshearing rate with respect to shearing stress as obtained throughout thelinear regime of the relationship. Expressed mathematically,

where the rate of shear, s, is expressed in centimeters per second percentimeter and the shearing stress I, is expressed in dynes per squarecentimeter.) These values of yield point and mobility have been derivedfrom measurements obtained on the Stormer viscosimeter modified asfollows:

A variable weight was used as the load on the driving pulley; a solidbrass cylindrical rotor was substituted for the standard rotor, thecylinder having a radius of 1.03 centimeter, a length of 3.20centimeters and a shaft diameter of 0.6 centimeter; and the standardbaffles cup was replaced by a jar having an inside diameter of about 8.5centimeters and filled to a depth of about 6 centimeters, the rotorbeing immersed in the coating material in the center of the jar to adepth of 1 centimeter on the rotor shaft. The force exerted at the rotorsurface wascomputed to be 5.23 dynes per square centimeter per gramapplied weight. The viscous shear, which is the linear rate of shear perunit distance between shearing planes or centimeters per second percentimeter, was determined by calibration of the instrument agagnstfluids of known viscosity, and was found to e where t represents theperiod of one revolution of the rotor.

That characteristic of our coating material which indicates that itfalls within the maximum and minimum limits of yield point and withinthe maximum and minimum limits of mobility hereinbefore expressed willhereinafter be referred to as body." Thus, coating materials heretoforeemployed having not less than 0.50 the mobility and substantially zeroyield point, have no body. Body may be expressed in the dimensions ofviscosity but differs from viscosity in that it takes into account bothmobility and a definite yield point or apparent yield point within apredetermined range, whereas viscosity contemplates the relationship ofthe rate or shear and shearing stress for a material having zero yieldpoint.

We have found that a coating material made in accordance with ourinvention has a body of about 5 to 150 seconds as measured on a Stormerviscometer modified as above, using a 500 gram load on the drivingpulley and expressed as the period of revolutions of the rotor, theoperations being performed at 40 C., and the expres-' sion body as usedin the specification and claims hereof shall be considered as definedherein.

The coating materials having a body of 5 to seconds contemplated hereinare of the type comprising a mineral pigment and an adhesive in anaqueous carrier. A suitable deflocculating or dispersing agent may beemployed, particularly where the type of mineral pigment is such that asuitable slurry cannot be made without it. Auxiliaries, or materials forspecial purposes may also be added to meet certain conditions. Thepigments which may be used comprise clay, calcium carbonate, calciumsulphate, barium sulphate, titanium oxide, zinc sulphide, satin white,zinc oxide, aluminum hydrate or mixtures thereof. The effect of theindividual pigments upon the body of coating material may difier, forinstance, some pigments, other conditions remaining the same, may tendto raise the body of the material more than others. The quantity ofpigment, in general, appears to have a greater effect upon the apparentyield of the coating material than upon the mobility thereof. However,it does to a proportional extent exert an influence upon both factorswhich constitute the body of the material.

Some types of pigments, such as refined coating clay, generally requirethe aid of a dispersing agent to form a defiocculated suspension inwater. Others, such as a water dispersing grade of titanium dioxide maybe suspended in water in a deflocculating condition without such aid.If, 7

however, a deflocculating agent is necessary, the deflocculating ordispersing agents contemplated comprise sodium silicate, tetra-sodiumpyrophosphate, di-sodium phosphate and tri-sodium phosphate, sometimessupplemented by an agent such as caustic soda, soda ash, or a soap, suchas ammonium stearate, sodium oleate or sodium stearate. In those typesof mineral pigments which contain residual coagulants, 0 to 6 parts ofdispersing agent per 970 quarts of mineral pigment (bone dry weight) hasbeen found to be satisfactory.

The body of the coating material may be controlled to a predeterminedextent by the solids content thereof, the solids content having apronounced efiect upon the apparent yield point of the pseudo-plasticcoating material, e. g., as the percentage of solids increases theapparent yield point increases.

The adhesive employed may comprise starch,

casein, soy bean protein, glue or mixtures thereof, and the amount ofadhesive employed relative to the remaining ingredients of the coatingmaterial greatly influences the mobility of the material. For example, ahigh viscosity starch will produce a coating of relatively low mobilityand vice versa, other factors being equal. Prepared starches can beobtained with a wide range of viscosities, but for control purposes itis advantageous to modify an untreated starch so as to obtain a pastewith the desired characteristics which may be varied at the discretionof the operator.

The control of mobility may also be exercised by the use of auxiliariesadded to the coating material. For instance, the viscosity of a.starchsized coating can be increased, thereby decreasing th mobility ofthe coating, through the use of various soaps and fatty acid or fattyalcohol derivatives, such as ammonium or sodium stearates, sulfatedfatty alcohols, sulfonated castor oils, or the like. A relatively smallamount of these materials is sufficient to produce a. large effect onthe mobility of the coating. The auxiliary material 'may be added to thecoating mixture either before or following the addition of the starch,although it is preferred to add it after the adhesive, since the largesteffect is produced in this way, and the mixing operation is simplified.

The amount of soap employed is logically based upon the adhesive, sinceits chief effects are the result of a definite interaction between thesoap and the adhesive. Preferably 2.0 to parts of soap based on 100parts of adhesive is employed. However, depending upon the resultdesired no soap may be used, or as high as 30 parts soap per 100 partsadhesive may be employed. When using starch as the adhesive, ammoniumstearate is the preferred soap, although the other auxiliarieshereinbefore set forth also produce desired resuits,

The kind of starch employed is a factor if the mobility of the coatingcomposition is to be controlled vby the effect of the auxiliaries uponthe starch, since not all starches are thickened to the same extent byadditions of soap. Raw tapioca or pearl starch, modified by the actionof enzymes is preferred, although chlorinated and other preparedstarches exhibit the same quality of sensitiveness to soap.

The adhesive content of the coatin material is also dependent to a largedegree upon the use requirements of the paper. A relatively large adhesive content, for instance, gives a high pick test, and low absorbencyfor printing inks and a low adhesive content gives a relatively low picktest, and a high degree of absorbency for ink.

The adhesive content of our coating material for paper ranges betweenabout 60 to 300 parts per 970 parts of pigment (bone dry) and thepreferred range is between 120 and 200 parts adhesive to 970 partspigment. Below about 60 parts adhesive solids to 970 .parts pigment thecoating material doesnot have the desired body hereinbefore specified,and above about 300 parts of adhesive solids to 970 parts pigment thecoating becomes more grease-proof and resistant to ink and thereforemakes the surface a poor one for printins.

Other agents when added to starch-sized coating mixtures may be employedto control the mobility of the coating composition. For instance,dispersions of casein, locust bean gum, soluble alginates, methylcellulose, ethyl cellulose and other water-soluble resins and solutionsof borax may be employed as mobility control agents.

For the purpose of the aforementioned highest speed roll or machinecoating operations as contemplated by the present invention and thesimultaneous production of fiat coatings of highly printable quality itis essential that our coatin material have a relatively high solidscontent, such as for example from at least about 50 to about 60-62 percent solids.

One of the advantages of such high solids content is that a heavierweight of coating can be applied per pass. For example with a 45% solidscontent, 6 pounds per side per ream can be applied, whereas-with a 60%solids content 13 pounds per side per ream may be applied.

A further advantage. as previously indicated, is that with such greaterdensities better films can be formed, or preformed, on the coating orapplicator roll which facilitates transfer therefrom and application tothe passing paper web. With higher solids content, and conversely lowerwater content, relatively thin films can be formed, the advantage ofsuch thin films being that they are smoother as distinguished from filmsof relatively higher water content which must be thicker to transferproperly and deposit a desired amount of solids and which consequentlyare conducive to undesirable formation of ridges.

Another and most important advantage is the ability to operate at theaforementioned high speeds. Thus, for example, with a solids content ofabout 53% the coating operation has been successfully conducted at alinear web speed of 930 feet per minute, as distinguished from thenecessity to slow down to, for example, a linear speed of about 750 feetper minute when utilizing a solids content of 45%, and to evenappreciably slower speeds with still lower solids content. It will beevident by the increase of solids content of 8% (between the 45% solidscomposition and the 53% solids composition) that a 24% increase in speedof production is obtained, without sacrifice in quality. Such increasein speed of coating enables the production of the paper web at like highspeeds, and a consistently high, and if desired continuous, speed ofcoating, or over all high speed production of coated paper, Thetremendous commercial and economical factors involved will be readilyapparent as a result of our improved coating composition, and viewed inthe light of the foregoing comparison the provision of a successfulcoating composition of at least about 50% solids content rendersoperations with compositions even of 45% solids content commerciallydisadvantageous under competitive conditions.

Added to the foregoing is the advantage of lower water content whichentails lower costs of evaporation, in addition to the operationalbenefits of ability of the web to readily absorb the requisite amount ofwater from the coating composition so as to prevent sticking on thedryers, which is a factor in the ability to conduct the coatingoperation at such indicated high speeds.

As a general rule it is desirable to have as high a body and solidscontent as possible and the upper limits herein expressed are considereding away from the coating rolls; the penetration of the coating materialinto the sheet is too slow causing a build-up of coating material on thedistributing and coating rolls and sticking of the coated sheet to thedriers; and at high speeds the problem of penetration is particularlyacute.

As examples of our coating composition the following are set forth forthe purpose of illustration, but not of limitation:

This coating material has a body of approximately 150 seconds; a totalsolids content of 56.6 per cent; an adhesive content of 140 parts basedon 970 parts of bone dry mineral pigment; a mobility of 0.09 rhe, and anapparent yield of 5130dynes per square centimeter. This coating materialis ideally adapted for the application of a relatively heavy coating toan absorbent sheet at relatively high speed.

Pounds of Pounds of Material Bone Dry Water Coating Clay 1,000 070 so25% Starch Paste 560 140 420 Tetra Sodium Pyro Phosphate (Anhydrous) 2.02.0 Water 400 400 Total 1,962 1,112 850 This coating material has a bodyof approximately 6.2 seconds; a solids content of 56.7 per cent; amobility of 0.28 rhe; and an apparent yield of 600 dynes per squarecentimeter. This coating material is ideally adapted for the applicationof a relatively heavy coating to a comparatively non-absorbentorresistant sheet at relatively high speed.

Pounds of Pounds of Material Bone Dry water Coating Clay 1,000 970 3025% Starch Paste 320 80 240 Water 700 700 Total 2, 020 1, 050 970 Thiscoating material has a body of approximately seconds; a total solidscontent of 52 per cent; and an adhesive content of 80 parts based on 970parts of bone dry mineral pigment. This coating material is ideallyadapted for the application of a relatively lighter coating to anabsorbent sheet at relatively high speed.

We claim as our invention:

1. A mineral coating composition adaptable for the coating of paper bythe machine coating method in a high speed paper making process whichcomprises, an aqueous dispersion of a mineral pigment, a defiocculatingagent for said pigment, from about 60 to about 300 parts of enzymemodified starch adhesive based on 970 parts of bone dry mineral pigment,and about 2 to 10 parts of ammonium stearate based on 100 parts Ti 8 ofsaid starch, said composition having asolids content of from at leastabout 50 to about 60-82 percent, an apparent yield point of between 500and 5,200 dynes per square centimeter and a range of mobility betweenabout 0.0575 and 0.750 rhe.

2. A mineral coating composition adaptable for the coating of paper bythe machine coating method in a high speed paper making process whichcomprises, an aqueous dispersion of a mineral pigment, a small amount.of tetra-sodium pyrophosphate as a deiiocculating agent for saidpigment, from about 120 to about 200 parts enzyme modified starchadhesive based on 970 parts of bone dry mineral pigment, and about 2 to10 parts of ammonium stearate based on parts of said starch, saidcomposition having a solids content of from at least about 50 to about60-62 per cent, and being of a thixotropic nature.

3. The process of coating paper webs in a continuous paper makingprocess which comprises, continuously spreading onto a rotating surfacea controlled amount of a mobile, dense, thin, smooth coating compositionfilm, said coating composition comprising an aqueous dispersion of amineral pigment, a deflocculatlng agent for said pigment, from about 60to about 300 parts of enzyme modified starch adhesive based on 970 partsof bone dry mineral pigment, and about 2 to 10 parts of ammoniumstearate based on 100 parts of said starch, said composition having asolids content of from at least about 50 to about 60-62 per cent, anapparent yield point of between 500 to 5,200 dynes per square centimeterand a range of mobility between about 0.0575 and 0.750 rhe, andtransferring said composition in the form of a preformed film from saidrotating surface to the surface of a paper web passing forwardly at alinear speed of from about 800 to about 1,000 feet per minute to providethe web with a highly printable smooth surface.

4. The process of coating paper webs in a continuous paper makingprocess which comprises, continuously spreading onto a rotating surfacea controlled amount of a mobile, dense, thin, smooth coating compositionfilm, said coating composition comprising an aqueous dispersion of amineral pigment, a small amount of tetrasodium pyrophosphate as a.deflocculating agent for said pigment, from about to about 200 parts ofenzyme modified starch adhesive based on 970 parts of bone dry mineralpigment, and about 2 to 10 parts of ammonium stearate based on 100 partsof said starch, said composition having a solids content of from atleast about 50 to about 60-62 per cent, and being of a thixotropicnature, and transferring said composition in the form of a preformedfilm from said rotating surface to the surface of a paper web passingforwardly at a linear speed of from about 800 to about 1,000 feet perminute to provide the web with a highly printable smooth surface.

5. A mineral coating composition adaptable for the coating of paper bythe machine coating method in a. high speed paper making process whichcomprises, an aqueous dispersion of mineral pigment, a small amount ofdeflocculating agent for said pigment, from about 120 to about 200 partsenzyme modified starch adhesive based on 970 parts of bone dry mineralpigment, and about 2 to 10 parts of ammonium stearate based on 100 partsof said starch, said composition having a solids content of from atleast about 50 to about 60-62 per cent.

6. A mineral coating composition adaptable for 9 the coating of paper bythe machine-coating method at a relatively high linear speed whichcomprises, a, mineral pigment and a modified starch in aqueousdispersion, the composition being of a thixotropic nature and having asolids content of from at least about 50 to about 60-62% and a starchcontent of about 60 to 300 parts based on 970 parts of bone dry mineralpigment.

7. A mineral coating composition adaptable for the coating of paper bythe machine-coating method at a, relatively high linear speed whichcomprises, a mineral pigment and a chlorinated starch in aqueousdispersion, the composition being of a thixotropic nature and having asolids content of from at least about 50 to about (50-62% and a starchcontent of about 60 to 300 parts based on 970 parts of bone dry mineralpigment.

8. A mineral coating composition adaptable for coating paper with athin, dense, printable surface film by the machine-coating method at apaper web speed of from about 800 to about 1000 linear feet per minutewhich comprises, a mineral pigment and a modified starch in aqueousdispersion, said composition being of a thixotropic nature and having asolids content of from at least about 50 to about 60-62% and a starchcontent of about 120 to 200 parts based on 9'70 parts of bone drypigment.

9. A mineral coating composition adaptable for the coating of paper bythe machine-coating method at a relatively high linear-speed whichcomprises, a mineral pigment and a modified starch in aqueousdispersion, the composition being of a. thixotropic nature and having asolids content of from at least about 50 to about (50-62%,

a starch content of about 60'to 300 parts based on 9'70 parts of bonedry mineral pigment and from about 1 to about 6 parts of pigmentdispersing agent based on 970 parts of bone dry mineral pigment.

10. A mineral coating composition adaptable for the coating of paper bythe machine-coating method at a relatively high linear speed whichcomprises, a mineral pigment and a modified starch in aqueousdispersion, the composition being of a thixotropic nature and having asolids content of from at least about 50 to about 60-62%, a starchcontent of about 60 to 300 parts based on 970 parts of bone dry mineralpigment and about 2 to 30 parts of ammonium stearate based on 100 partsof starch.

11. The process of coating paper webs in a. continuous paper-makingprocess which comprises, continuously spreading onto a rotating surfacea controlled amount of a mobile, dense, thin, smooth film of a, coatingcomposition of thixotropic nature, said coating composition comprisingmineral pigment and modified starch in aqueous dispersion having asolids content of from at least about to about -62% and a starch contentof about 60 to 300 parts based on 9'70 parts of bone dry mineralpigment, and transferring said composition in the form of a preformedfilm from said rotating surface to the surface of a paper web passingforwardly at a linear speed of from about 800 to about 1000 feet perminute.

12. Paper coated with the dry residue of a film of coating compositionin accordance with claim 5.

13. Paper receptive to fine printing characterized by smooth mineralpigment coating of substantially uniform texture and flatness, compactedand bonded to the paper web surface, said coating being the driedresidue of a film of coating composition in accordance with claim 6.

GILBERT K. DICKERMAN. ROBERT W. RILEY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,194,216 Coppock Mar. 19, 19402,185,859 Massey Jan. 2, 1940 2,124,372 Kesler July 19, 1938 2,293,690Harrigan Aug. 18, 1942 2,024,123 Baker Dec. 10, 1935 FOREIGN PATENTSNumber Country Date 496,775 Great Britain Dec. 6, 1938 OTHER REFERENCESDe Guehery, Technical Association Sec. of Paper Trade Journal June 30,1938, pages 76-77.

